Depreciation is the allocation of the cost of a non-current asset over the useful life of that asset. The basic principles of depreciation involve recognizing the decrease in an asset's value over time due to usage, wear and tear, or obsolescence. The primary objective of depreciation is to match the cost of the asset with the revenues it generates during its useful life. The most common methods for calculating depreciation are:

1. Straight-line Method: This method spreads the depreciable amount of the asset evenly over its useful life. The annual depreciation expense remains constant.

2. Reducing Balance Method (also known as Double Declining Balance): This method assumes a higher rate of depreciation in the early years and gradually decreases over time, resulting in a higher total depreciation expense in the earlier years of the asset's life.

3. Units of Production Method: With this method, depreciation is calculated based on the asset's usage, rather than time elapsed. It's particularly useful for assets whose usage varies with production output, like machinery in a factory.

4. Sum-of-the-Years'-Digits' Method: This method front-loads the depreciation expense, with a higher rate in the early years, and then reduces as the asset ages.

5. Sinking Fund Method: This approach considers the future replacement cost of the asset and calculates the annual depreciation expense needed to accumulate sufficient funds in a sinking fund to replace the asset at the end of its useful life.

Each method has its own advantages and disadvantages, and the choice depends on factors like the nature of the asset, the pattern of benefits derived from it, and management's preference. The selection of the depreciation method should be disclosed in the notes to the financial statements.